Countless patients are suffering from severe acute or chronic pain, including pains associated with cancer and surgical operations. Pain treatment remains a serious clinical challenge. The current analgesic opioid drugs have side affects such as drug addiction, respiration restraint, stomach peristalsis reduction etc. Therefore, their clinical use is limited. There has always been a great need for novel centrally-acting analgesics having potent analgesic activities and clinical safety without the above side effects.